(noun) - From the Latin for "gap," a lacuna is a blank spot. In art, it may occur in a painting when the paint has ceased to adhere to the support. Think of Leonardo's famous Last Supper, in which the paint began to flake off the wall almost upon completion.
A lacuna may also appear on a painted object, such as a small ceramic sculpture that has been handled so often it has lost patches of surface color. The term is additionally used to describe a missing section of tesserae in a mosaic.
It is important to note that while a lacuna is most often the result of damage or degradation, it can be intentional. For as long as painters have used supports (canvases, wooden panels, or walls) prepared with white grounds (coatings, like gesso, that prepare a surface for painting), they've had the option to simply let the white show. It can pop as a highlight or save time in areas that need white. With the advent of Impressionism, and going forward, these white lacunae became and remain integral parts of many compositions.
Finally, this word is not confined to the visual arts. A lacuna may also refer to a gap -- caused by damage or a lost portion -- in a manuscript. It's not uncommon to see reference to such in historic documents, and these can cause fits to researchers. And lacuna is the correct term for a lengthy pause in a piece of music. You know the kind: a pause that is so pronounced some members of the audience will begin to applaud hesitantly.
luh·cue·nuh (singular), luh·cue·nee (plural)
gap, blank, space, opening, "an 18.5-minute void in the Nixon 'Watergate' tapes"
lacunae or lacunas (plural forms)
"He had read a good deal, too, but he had never forced himself to read anything that did not appeal to him, and so he was far too self-centered in opinion, with curious lacunae of astounding ignorance." -- Frank Harris, My Life and Loves, Volume 2 (1925)